The invention relates to a process for the production of a buckling-resistant stove-finished structural member from cold rolled and dressed strip (cold strip) from non-ageing steel with high bake-hardening potential, more particularly of more than 70 N/mm2.
To achieve a high bake-hardening potential as a rule use is made of steels which contain dissolved nitrogen as well as dissolved carbon. Examples of these are unkilled steels. The storage of strips of such steels at room temperature leads even after the short time of one or two days to ageing processes which make impossible any satisfactory further processing, more particularly cold working. There is also an adverse effect on the surface texture of the cold strips.
Ageing can be caused by the diffusion of dissolved carbon and/or nitrogen. In the case of pure carbon ageing the effect of temperature on ageing time can be estimated as follows: The times t1 and t2 required for identical ageing effects stand in converse ratio to the associated temperature-dependent coefficients of diffusion of carbon in xcex1 iron.
In the temperature range up to 100xc2x0 C. we therefore have                                                                                           t                  1                                ⁢                                  xe2x80x83                                ⁢                                                      (                                          T                      1                                        )                                    /                                      t                    2                                                  ⁢                                  xe2x80x83                                ⁢                                  (                                      T                    2                                    )                                            =                              xe2x80x83                            ⁢                              D                ⁢                                  xe2x80x83                                ⁢                                                      (                                          T                      2                                        )                                    /                  D                                ⁢                                  xe2x80x83                                ⁢                                  (                                      T                    1                                    )                                                                                                        =                              xe2x80x83                            ⁢                                                exp                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  -                        21.1                                            /                                              (                                                  1                          ,                                                      987                            ·                                                          10                                                              -                                3                                                                                      ·                                                          T                              2                                                                                                      )                                                                                                              exp                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  -                        21.1                                            /                                              (                                                  1                          ,                                                      987                            ·                                                          10                                                              -                                3                                                                                      ·                                                          T                              1                                                                                                      )                                                                                                                                                    [        1        ]            
with T1,2 in K.
Table 1 shows the factors calculated according to equation [1] for the delay in time of an ageing effect due to lowered temperatures. For example, in comparison with ageing at room temperature, ageing at xe2x88x9210xc2x0 C. lengthens ageing time by 62 times.
The quantity of description of the effect of dissolved nitrogen on steel ageing can be carried out similarly to the description of carbon ageing according to equation [1], using the coefficient of diffusion for nitrogen. The connection between ageing time and ageing temperature is therefore obtained as follows:                                                                                           t                  1                                ⁢                                  xe2x80x83                                ⁢                                                      (                                          T                      1                                        )                                    /                                      t                    2                                                  ⁢                                  xe2x80x83                                ⁢                                  (                                      T                    2                                    )                                            =                              xe2x80x83                            ⁢                              D                ⁢                                  xe2x80x83                                ⁢                                                      (                                          T                      2                                        )                                    /                  D                                ⁢                                  xe2x80x83                                ⁢                                  (                                      T                    1                                    )                                                                                                        =                              xe2x80x83                            ⁢                                                exp                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  -                        18.33                                            /                                              (                                                  1                          ,                                                      987                            ·                                                          10                                                              -                                3                                                                                      ·                                                          T                              2                                                                                                      )                                                                                                              exp                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  -                        18.33                                            /                                              (                                                  1                          ,                                                      987                            ·                                                          10                                                              -                                3                                                                                      ·                                                          T                              1                                                                                                      )                                                                                                                                                    [        2        ]            
with T1,2 in K.
Table 2 shows the factors calculated according to [2] for the delay in ageing effect caused by dissolved nitrogen.
It is an object of the invention to provide a process for the ageing-free further processing of cold strips of an ageing-sensitive steel with high bake-hardening potential to produce a stove-finished structural component.
To resolve this problem the invention provides a process as set forth in claim 1 or a process as set forth in claim 3.
In the process according to claim 1 the ageing of dressed cold strip is suppressed by its storage at low temperature. In the alternative process set forth in claim 3, due to the bake-hardening effect triggered thereby the stove-finishing performed shortly after further shaping processing prevents the ageing of the cold strip dressed shortly prior to further processing.
To make use of the positive effect of a lowering of the surrounding temperature during the storage of cold strips, the storage temperature T in K (degrees Kelvin) can be estimated as follows, in dependence on the planned storage time in hours:
xe2x80x83T=9225/(31.48xe2x88x921n (48/t)xe2x80x83xe2x80x83[3]
Equation [3] follows from equation [2] and relates to a steel which can no longer be satisfactorily processed, due to nitrogen ageing after exceeding a storage time of more than 2 days at 20xc2x0 C. In the case of ageing by both elements, it is enough to allow for nitrogen only, due to the lower diffusion speed of carbon in comparison with nitrogen.
As an example, the change in material properties due to ageing at different temperatures was measured on a cold strip of a steel containing 0.003% C, 0.27% Mn, 0.003% Si, 0.007% P, 0.006% S, 0.046% A1, 0.001% N and Cu+Ni+Cr less than 0.1% (values in % by weight). After hot and cold rolling the steel was galvanised in a continuous fire-coating installation with a maximum annealing temperature of 820xc2x0 C. and then subjected to 1.5% dressing. The difference between the upper and lower yield points (Rehxe2x88x92Rel) was evaluated from the tensile test as a measure of the risk of stretcher strains.